The present invention relates to a convergence correction circuit of a display using a CRT, and in particular to such a convergence correction circuit suitable to a multiscan projection display capable of handling with a plurality of different deflection frequencies.
In a projection display, three projection tubes comprising red, green and blue tubes are typically arranged in a horizontal direction together with a projection lens to project and synthesize a color image onto a screen. Since red and blue images are projected onto the screen in a slant direction, a keystone distortion as shown in FIG. 2 is generated on the basis of projective geometry, resulting in a color shift
This color shift has heretofore been corrected by mounting a convergence yoke (hereafter abbreviated as CY) onto the neck section of a projection tube. The CY being similar to a deflection yoke, supplementally deflects an electron beam by applying the output of a convergence amplification section to the CY. A correction signal applied to the CY is desired to be synchronized to the deflection scanning period and be capable of correcting a color shift pattern with high precision. There are currently known correction waveform generating means of the analog scheme types and digital scheme. Although the analog scheme is simple, it has a drawback of coarse precision. The digital scheme has high correction precision, however, it has a drawback of high cost because a large memory capacity is needed. A method for reducing the needed memory capacity is described in U.S. Pat. No. 4,422,019 issued in 1983. In accordance with that method, the screen is represented by approximately 16.times.16 representative points, and only the correction information of those representative points is memorized while the remaining areas are interpolated using data of the representative points. In a convergence signal generating circuit described in the U.S. Pat. No. 4,422,019, interpolation in the horizontal direction of the screen is performed by using a conventional low-pass filter, while interpolation computation in the vertical direction is performed in a digital or analog way to generate a correction signal. Such prior art is primarily suitable to one specific scanning format. However, it has a drawback that a large memory capacity is needed in application to a so-called multiscan display capable of displaying formats of a plurality of schemes which are different in horizontal scanning frequency and screen size. Even if correction is completed in one format of the multi-scan display, for example, a color shift is generated when the horizontal frequency of the display is somewhat changed, resulting in a problem
Further, in case a convergence correction signal is generated in a projection display of the prior art, correction waveforms synchronized to the deflection scanning period must be generated in any case, although red is opposite to blue in polarity.
In a conventional technique, the horizontal deflection current is detected as a voltage signal by using some means, and waveforms are synthesized on the basis of the resultant voltage signal.
This conventional scheme has a problem that a color shift remains in the left end portion of the screen. The reason will now be described. The above described waveform synthesis processing needs a finite delay time. The delay time typically has a value equivalent to approximately 2 to 3% of a horizontal period. Since a picture of a CRT is formed by the horizontal scanning and the vertical scanning, the left end portion of the screen corresponds to the position immediately after the high-speed horizontal retrace line. Since the correction signal information reaches there late, the color shift remains in a portion located at the left end portion of the screen and occupying approximately 2 to 3% of the total screen width.
Further, in the conventional scheme, if the deflection frequency changes, the voltage amplitude and phase of a parabolic signal changes, resulting in deterioration of picture quality.